Flow, Turbulence and Combustion最新文献

Hydrogen/methane (H₂/CH₄) blends serve as a transitional fuel for gas turbines, offering a pathway toward cleaner energy by reducing carbon emissions while leveraging existing natural gas infrastructure. Understanding the effects of H₂ content and pressure on the flame shape is essential for safe operation and optimising combustion performance. In this work, turbulent lean premixed Bunsen H₂/CH₄ flames are studied using large eddy simulation (LES), focusing on the flame brush length (f_l) and thickness (f_b) along the centreline. The results have been compared with available measurements for validation. The findings indicate that increasing H₂ content leads to shorter and thinner flames, while the pressure effects are minimal. At the lower H₂ content (≤30% by volume), the change in f_l is relatively small, whereas at the higher H₂ content ( > 30% by volume), f_l decreases linearly with increasing H₂ content. Similarly, the values of f_b decrease as H₂ content is increased. This phenomenon is primarily due to the increased turbulent flame speed (s_T) resulting from enhanced fuel reactivity. The limited pressure effects on f_l and f_b are attributed to the consistent s_T, maintained by the synergistic interactions between chemical reactions and turbulent characteristics. Furthermore, this study highlights the transition of the flame regime from the corrugated flamelet regime to the thin reaction zone regime along the axial direction. This transition occurs further upstream in flames with higher H₂ content, driven by the increased flame speed. Additionally, the reaction progress variable contours and profiles show that the H₂ addition also leads to a radial thinning of the flame, while pressure has little effect on the overall flame shapes.

Scramjet External Nozzle (SEN) generally has a single expansion ramp wall in one side and the other side wall called cowl is fully or partially truncated. These designs are to aim large expansion area using rear part of a vehicle airframe with reduction in system weight, friction loss, and heat load. The present study experimentally investigated the effect of cell base width on the thrust performance of the SEN and computationally expressed those effects, based on the wind tunnel test. The cell base structure divides each engine module. Three test models were employed of which shape differs from each other. One is not clustered configuration, while the others are clustered and each of them has a different cell base width. The experimental study showed that the nozzle wall pressure distribution varies corresponding to the cell base width and optimum-expansion condition is most sensibly affected by cell base width. Additionally, the input correction is proposed to reflect the clustering effect to the computation model, which unifies the input physical quantities by solving mass, streamwise momentum, and energy conservation equations. The effectiveness of such a correction was confirmed using so-called Easy-to-Handle Prediction Model (EHPM) for SEN which is based on the wave method. Comparison between calculation and validation test results demonstrated that the prediction error could be suppressed under the threshold of 10% through the proposed correction.

Direct numerical simulations with multi-step chemistry were performed for one- and two-dimensional freely propagating laminar premixed flames of methane–air and hydrogen–air mixtures with a matching density ratio to isolate the effects of hydrodynamic instability while allowing for a variable effective Lewis number, with the methane (hydrogen) flame being thermodiffusively stable (unstable). Entropy diffusion and generation mechanisms were analysed based on contributions from heat conduction, viscous dissipation, mass diffusion, and chemical reactions. Across both flames, chemical reactions were identified as the dominant source of entropy generation, with viscous dissipation contributing negligibly compared to other mechanisms. Significant differences were found in the structure of entropy generation rates across both flames, with varying degrees of correlation with curvature. Stronger correlations were found between the irreversible entropy generation rates and the heat release rate in both flames, suggesting the former as a potential marker for thermodiffusive instability. Analysis of the entropy generation profiles at representative locations across a flame front further revealed possible origins of the entropy behaviour under thermodiffusively stable and unstable conditions.

Floating wind energy is a relatively new area that consists of harnessing wind energy from wind turbines that are supported by a floating foundation. This enables the installation of offshore wind turbines in deep seas, which means tapping into offshore wind resources that are unreachable with bottom-fixed wind turbines. Up to now, the feasibility of floating wind turbine technology has been demonstrated in small pilot farms. However, floating wind turbines are still subject to unexpected failures. Therefore, a better fundamental understanding of these turbines is needed to improve the technology to accelerate its deployment and reduce the cost of energy. Furthermore, the dynamics of floating wind turbines is different from those of their bottom-fixed counterparts. This presents challenges and opportunities across the different phases of their development and operation. This position paper addresses the fluid mechanics community and presents key challenges and research needs in the field of floating wind energy. Building on the grand challenges identified in the wind energy community, the manuscript addresses three focus areas and their interactions: the met-ocean conditions, the wind turbine, and the wind farm. Five groups of fluid mechanics driven challenges are highlighted: unsteady aerodynamics, high-speed flows, non-linear hydrodynamics, flow-induced vibrations, and wake dynamics. In addition, the kind of research methods and infrastructure needed to address these challenges are discussed, including cross-cutting themes such as digitalisation and co-creation across stakeholders and disciplines. Finally, the conclusions provide overarching recommendations to solve the upcoming challenges in floating wind energy and highlight the role that the fluid mechanics community could play.

Flow around rotating cylinders with dimples has been evaluated at a Reynolds number of Re = 2000. It has been determined since the rotational influence was more effective for lower Re values. Angular dimple distributions have been considered as 15° ≤ β  ≤ 60°. Different rotation rates have also been evaluated from α = 0 to α = 1.26 as lower rotation rates for the current Re were significant to suppress the vortex shedding. Particle Image Velocimetry (PIV) results have been compared by those of a bare cylinder. The wake regions have been shrunk due to dimpled surfaces. Excluding the case of α = 0.42 for a bare cylinder (4.5%), the regions had tendency to approach the bodies. The highest displacement (-78.9%) to the cylinder has been attained for β = 15° with α = 1.26 as observed. The lowest value (-5.6%) has been obtained by α = 0.42 for β = 45° by approaching the cylinder. For the influence of passive flow control, the cases without rotation have been regarded and β = 60° was more dominant. For the wake lengths, the changes for F and S locations have also been exhibited in the wake regions as these points affecting the length of recirculation bubble were substantial. As interpreted from the vorticity results, peak values were more dominant in the cylinder wakes. Since the rotation rates were increased, these regions tended to shrink. It is related to the presence of fluctuations in the wake regions. The displacements of maximum turbulence kinetic energy values have been shown for the level of turbulence intensity. Turbulent intensity has been enhanced in the regions closer to the bodies except the stationary cases of β = 30° and β = 45°. Nevertheless, the forced rotation was more effective.

The accurate prediction of Laminar Separation Bubbles (LSBs) is crucial for understanding transition mechanisms and aerodynamic performance of airfoils operating at low Reynolds numbers. This study employs the Linearized and Segregated Variational Multiscale (VMS) Method to simulate LSB formation and development on airfoils. The methodology is validated against two well-documented test cases: SD7003 airfoil ({boldsymbol{Re}}=60,000), (boldsymbol{AoA}={4}^{circ}) and the E387 airfoil at ({boldsymbol{Re}}=3times 10^{5}), (boldsymbol{AoA}=1^{circ}). The results demonstrate strong agreement with reference data from the literature, accurately capturing pressure and skin friction distributions as well as velocity profiles associated with LSB dynamics. To further assess the method’s predictive capability, simulations are performed on a DU89-134 airfoil, designed for High Altitude Pseudo Satellites (HAPS) applications at ({boldsymbol{Re}}=5times 10^5), (boldsymbol{AoA}=1^{circ}), 5^∘. Comparisons with experimental vertical velocity profiles - obtained with hot-wires anemometry - show excellent agreement, confirming the method’s robustness in transitional flow modelling. These findings highlight the effectiveness of the Linearized and Segregated VMS approach for Large Eddy Simulations of LSBs. The method provides a reliable tool for aerodynamic analysis of low-Reynolds-number airfoils, with potential applications in next-generation high-altitude flight vehicles.

The rotating detonation engine (RDE) is an emerging propulsion concept with significant potential for advanced power generation and aerospace applications. This study presents three-dimensional (3D) numerical simulations of hydrogen–air RDE flow fields to elucidate the underlying mechanisms, explore control strategies, and identify characteristic parameters associated with rotating detonation waves (RDWs) across different operational modes-defined by the number of detonation waves propagating within the combustor. Various operational behaviors are investigated through a multi-ignition approach and by modulating two key parameters: the injection stagnation pressure ({p_{{rm{t}},{rm{in}}}}) and the injection area ratio ({A_{{rm{th}}}}/{A_{{rm{ch}}}}). Our findings show that different control operations result in self-regulating processes with varying fluctuation amplitudes. Specifically, increasing ({p_{{rm{t}},{rm{in}}}}) causes significant fluctuations, while increasing ({A_{{rm{th}}}}/{A_{{rm{ch}}}}) leads to smoother self-regulation. Moreover, ({A_{{rm{th}}}}/{A_{{rm{ch}}}}) is identified as a key factor for the RDE to achieve a positive pressure gain. The concept of non-confinement rate (delta ) is introduced to adjust the critical height value ({h^{bf{it{*}}}}) of RDWs, aligning with experimental results. A new equivalent available pressure (EAP) evaluation method based on the total enthalpy, i.e. EAPh, is also preliminarily introduced.

Turbulence is pivotal in many processes which are critical to engineering systems. High-fidelity computational fluid dynamics (CFD) simulations are increasingly used to aid the design of these systems. Representative inlet boundary conditions are needed to capture accurate fluid behaviour. Due to high computational costs, simulations often focus only on regions of interest, necessitating approaches that can accurately reproduce inlet turbulence fluctuations in truncated domains. The proper orthogonal decomposition and Fourier Series (PODFS) method provides a convenient and efficient way of compressing turbulence data from a precursor simulation for use as inlet data. A subset of POD modes provides the spatial information with significantly less overhead than recycling methods while Fourier series representations of the temporal components guarantee temporal continuity and flexibility in timesteps. We evaluate PODFS for a turbulent bluff-body flow and an effusion cooling rig. In the bluff body flow, large-scale vortex shedding and small-scale turbulence are present. The PODFS inlet is seen to reproduce the flow features downstream of the bluff body and the number of modes necessary for this is investigated. For the heat transfer case, inlet turbulence is known to be critical to the predictions of cooling effectiveness. PODFS based on a single simulation of the full jet in cross flow turbulence generator is used to provide the inlet conditions for cases using different blowing ratios. The correct trends with blowing ratio and inlet turbulence are reproduced, showing a significant saving in computation time over using the full geometry in each case.

This work investigates cycle stability issues associated with the operation of a constant volume combustor concept applied to a pistonless gas turbine in the context of pressure gain combustion. Experimental tests are carried out on a lab-scale combustion vessel designed for cyclic operation. This facility features the turbulent combustion of air and fuel direct-injection, with a controlled overall equivalence ratio. Operating conditions are set to reach either spark-ignited flame propagation or self-ignition induced by residual burned gases or residual flame. Self-ignition events develop over two consecutive cycles, where the first (spark-ignited) cycle acts as a preparatory phase. During this phase, a slow flame propagates through the exhaust phase, establishing the conditions necessary for self-ignition in the following cycle. Chemiluminescence recordings of OH* and CH* radicals are performed to shed light onto the phenomenology of self-ignition processes. A thermodynamic model of constant volume combustion cycle is implemented to further analyze the energetic behavior of spark-ignited versus self-ignited cycles. Through the model, it is found that self-ignited cycles exhibit approximately 15% less total heat losses compared to the reference spark-ignited configuration.